feat: persistent-pipeline reuse — push_blocking, node introspection, stateful wrapper

Adds three pieces needed to build one KPN network and reuse it across many
replays/configs instead of tearing down and rebuilding per run:

- Channel<T>::push_blocking (+ IVariantChannel/VariantChannel forwarding):
  lossless backpressure push that waits for space instead of dropping when
  the ring is full. PyNode's run_loop now uses it so a downstream consumer
  lagging behind never silently drops a frame.
- PyNetwork::node_ptr / node_stats: raw node handle by name (for a binding
  to dynamic_cast to a concrete wrapper and call functor-specific runtime
  setters) and a per-node timing snapshot for profiling.
- ObjectVariantNodeWrapper: variant-node adapter for functors that need
  runtime-constructed state (a Config, a loaded gallery), mirroring
  VariantNodeWrapper's channel plumbing but backed by ObjectNode<Obj>.

Built and used downstream in scene-actor-extraction's sae_kpn Python replay
bindings for repeated threshold-sweep evaluation of the same pipeline.

include/kpn/channel.hpp

@@ -136,6 +136,35 @@ public:
             push_callback_();
     }
 
+    // Lossless push with BACKPRESSURE: if the ring is full, wait for the consumer to
+    // drain instead of dropping (the throwing push()) — the producer just runs slower.
+    // Use when every value must be delivered (e.g. replaying a dump for scoring, where
+    // a dropped frame silently corrupts the result). SPSC: only the sole producer may
+    // call it. Returns false if the channel was disabled while waiting.
+    bool push_blocking(T value) {
+        for (;;) {
+            if (!accepting_.load(std::memory_order_acquire)) {
+                stats_.record_drop();
+                return false;
+            }
+            const std::size_t t = tail_.load(std::memory_order_relaxed);
+            const std::size_t h = head_.load(std::memory_order_acquire);
+            if (t - h < capacity_) {          // space available → normal push
+                const std::size_t data_bytes = ChannelDataSize<T>::bytes(value);
+                const bool was_empty = (t == h);
+                buf_[t & ring_mask_] = make_storage(std::move(value));
+                tail_.store(t + 1, std::memory_order_release);
+                stats_.record_push(t - h + 1, data_bytes);
+                wake_.fetch_add(1, std::memory_order_release);
+                wake_.notify_one();
+                if (was_empty && push_callback_) push_callback_();
+                return true;
+            }
+            // full: yield briefly and retry (consumer will drain)
+            std::this_thread::sleep_for(std::chrono::microseconds(50));
+        }
+    }
+
     // Lossless, non-blocking delivery for a must-deliver control token (EOF).
     //
     // A sentinel is stored out-of-band — in a dedicated slot that does NOT